I’ve opened 3 Mysterious chests separately in Mysterious caves level 1 and there were only to blue items in them. There was no liquid rainbow. What should i do?
*** I have a video of the third chest to proof my statement
https://drive.google.com/file/d/11Eo6P0aSqqBlRSYzltk7NLFWYINGGkjG/view?usp=drivesdk
Did you find the chest on the first or second floor of the cave? It looks like a normal Resplendent Chest but it’s always on the first floor of the cave though you may not see it if you’re simply on the bounty to “Clear the Mysterious Cave” which only cares about the second floor.
This is the Bug Report Forum. You should ALWAYS include screenshots and/or videos.
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Small correction: the Mysterious Chest can spawn on the 2nd floor, even if it appears most often on the 1st floor.
Really? Wow… in years of playing and farming for that staff for every season since the altar came along on multiple servers (mostly NA and EU) I’ve never found it on the second floor. Thanks for the tip! I wonder if I’ve seen the chest and just ignored it because I was in a hurry. LOL, wouldn’t that be a kick in the pants?
I will post the video. Thanks.
Is it so that if one already has the liquid rainbow then it won’t drop again?
On this season never searched or found it before. The last time I searched it was in season 28.
My thought was that if the rainbow is a guaranteed drop from the mysterious chest, but only if one doesn’t have it already, then the game mistakenly has noted you for having obtained it. Which is bad of course.
You could at least check at the blacksmith to see what ingredients you already have for the staff.
Liquid Rainbow will drop even if you already have one.
Good to have that one cleared up Boubou 
Am I mistaken about it usually being a guaranteed drop?
It is… on the PC.
OP is playing on a Console.
You will recall that Consoles don’t have the Gibbbering Gem… perhaps the Liquid Rainbow is the equivalent for the Gibbering Gem there – you need to have the right dungeon + the right chest + the drop. Don’t know… I don’t play on Consoles.
Maybe OP should post in the console bug report forum then
(Not criticising you Boubou of course).
It looks like you’re playing on console. (@Boubou caught that). This Bug Report Forum is for reporting D3 game bugs on a PC on the Live server. You should move this thread to the Console Bug Report Forum.
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This is true for all the Staff of Herding components on PC. (Except maybe the Plan. I’m not sure about that one).
Console players do not need all of the same components as PC players to craft a Staff of Herding. (Also pointed out by @Boubou).
(Sorry for repeating what others had posted. I missed several posts since last night. So, I was commenting as I was reading down the list).
Yep… the plan drops only once. If you already have it, or learned it, then it won’t drop.
There’s an old trick… a useless one I think… if you give the plan to another hero, it will drop again if Haedrig hasn’t learned it. But why would anyone do that – it is « account bound » ? 
It drops all the time when I hit the cave must be bad luck.
The mysterious chest is not guaranteed to show up when you get access to the cave via Zaven.
This needs to be moved to Console Bug Report… so PC players don’t comment on it.